For efficient purification, therefore, some method of reducing and modifying the character of organic solids, particularly those in solution, must be selected. In seeking a method by which this may be accomplished, scientific men found years ago that this very process was being carried on continually by natural forces, although at a very slow rate of purification. All organic matter, however formed and wherever present, is subject to the natural forces of decay. Fruits, vegetables, and meats of all kinds, exposed to the air, rapidly lose their original character and form and in the course of time disappear entirely. Except for this provision of nature, the accumulation of organic wastes since the beginning of the earth’s occupation by human beings would be so great that the earth would be uninhabitable on account of the deposits of waste matter which would have formed by this time. Nature, then, recognizes the need of disposing of organic wastes, and her method is the one which apparently must be followed by human beings if successful treatment is to be secured. Only a few decades ago, it was found that this process of decay was due to the activity of very small organisms known as bacteria, and their agency was proved by experiments which showed that if vegetables or meat were kept free from bacteria, no decay, fermentation, or putrefaction took place. It was proved that the air itself was not responsible because in certain experiments air was allowed to enter through a filtering medium fine enough to strain out the bacteria and no decay took place, although oxygen and air were both freely admitted. It is well understood by the housewife that fruits can be kept indefinitely if they are cooked sufficiently to kill any bacteria present and then sealed in bacteria-free, air-tight jars. When such preserves spoil, it is because some bacteria were left in the jar or have since been admitted through an imperfect top. When decay is allowed to proceed, the obvious result is, first of all, a softening of the material, as in the case of a rotten apple, a liquefaction, as it is more technically known. Following that part of the process is a gradual breaking down of the material, the residue being of an earthy character which is assimilated by the soil into which it falls.